The invention relates to method for level control in a pneumatic level control of a motor vehicle having at least two axles, at least one air spring per axle, if appropriate one or more pressure accumulators, at least one pressure sensor, means for determining the distance between at least one wheel and/or axle and the vehicle body, a control unit and a compressor. The axles are successively adjusted from a starting level to a desired level. During a positive-adjustment operation, at least from time to time the compressor is operated and/or a connection is established between at least one air spring and the pressure accumulator. The positive-adjustment operation of the air spring or air springs on an axle from a starting level to a higher desired level is interrupted by the control unit if at least one switch-off condition of the compressor has been fulfilled and/or the pressure level in the pressure accumulator is too low.
A method for level control in a pneumatic level control system of a motor vehicle of the type described in the introduction is known for example from DE 196 21 946 C2. This document describes a pneumatic suspension of a motor vehicle which allows the level of the vehicle to be raised or lowered in situ or to be kept at a predetermined level irrespective of the load level. A control unit which switches the compressor on and off comprises a computer, which calculated an estimated value for an operating temperature of the compressor and switches it off if the estimated value exceeds an upper threshold value. The control unit or computer switches the compressor on again if the estimated value drops below a lower threshold value. In each case the last estimated value is increased by a predetermined temperature rise when switching on the compressor; the degree of this predetermined temperature rise is dependent on the magnitude of the estimated value. While the compressor is operating, the estimated value is increased by a positive gradient, and when the compressor is inoperative it is reduced by a negative gradient.
DE 43 33 591 A1 has disclosed a motor controller which switches off the motor or the driven compressor for a predeterminable time based on the total on time and thereby protects the motor/compressor from overheating.
DE 40 30 475 A1 has disclosed a method for controlling a motor in which the starting point is the rising or falling temperature characteristic curve of the motor during operation or standstill, respectively. The method involves estimating an instantaneous value for the temperature of the motor by monitoring the operating time or the time for which the motor has been inoperative. If the estimated instantaneous value exceeds a predeterminable value, motor operation is stopped and an alarm signal is emitted.
The prior art has disclosed methods for level control in level control systems for motor vehicles which can be used to estimate the temperature of a motor or a compressor during operation without exceeding a predetermined limit value. This allows the compressor to be reliably protected from overheating. It is also known to fill an air spring with compressed air from a pressure accumulator and thereby to raise the level of the vehicle. The situation may arise whereby the pressure level in the pressure accumulator is insufficient to fill the air spring with the required volume of air which is necessary for the desired level to be set. This situation arises, for example, if the pressure level in the pressure accumulator is equal or approximately equal to the pressure level of the air spring and there is no longer any exchange of air between the pressure accumulator and the air spring. It is known from the prior art to combine the positive adjustment method with compressor and pressure accumulator, so that the pressure level in the pressure accumulator can be increased again by the compressor.
The methods described above have the drawback that a positive adjustment operation to a predetermined desired level is interrupted if the limit or threshold value is reached. Since the axles of a vehicle are normally adjusted successively from the starting level to the desired level, the situation may arise whereby one or, in particular in the case of trucks, more axles have reached the desired level, whereas the axle which is yet to be adjusted no longer reaches the desired level, on account of the switch-off condition being triggered, leading to the motor vehicle adopting a leaning position. To compensate for the leaning position, it is known from the prior art for all the axles to be adjusted negatively down to the starting level.
To prevent the motor vehicle from leaning, it is known from the prior art to lower the level at all the axles to the starting level, so that the original level before the positive adjustment operation commenced is set. While the compressor operation is switched off, the compressor can cool down, which can take a very long time under certain circumstances. The positive adjustment operation is restarted either automatically or based on a request from the driver (actuation of a button) if a switch-on condition of the compressor is fulfilled. In unfavourable situations, for example under extreme ambient conditions, it may be the case that the cooling of the compressor based on the switch-on condition is insufficient to adjust the motor vehicle from the starting level to the desired level, and a further positive-adjustment operation is again interrupted by the switch-off condition.
It is an object of the invention to provide a method for controlling the level in level control systems for motor vehicles which lowers the energy consumption, reduces the switching frequency of the corresponding components and allows the desired level to be reliably reached even under extreme ambient conditions.